The present invention relates to an improvement in the selective production of .alpha.-olefins from synthesis gas. More particularly, the invention concerns reaction of synthesis gas in the presence of a unique catalyst system comprising palladium, iron, and zinc.
The preparation of hydrocarbons from synthesis gas (i.e., mixtures of carbon monoxide, hydrogen, and sometimes other gases, such as carbon dioxide, nitrogen oxides, etc.), has been the subject of extensive investigation. During World War II, the use of iron-based catalysts for this reaction was developed in Germany. An extensive review of this work has been provided by Storch et al. in the book entitled "The Fischer-Tropsch and Related Syntheses" (John Wiley & Sons, 1951). In general, these catalysts produced a broad range of hydrocarbons under operating conditions.
The serious disadvantage of most such processes has been the non-selective nature of the product distribution. Catalysts having acceptable activity often tend to give a wide spectrum of products. This lack of selectivity not only complicates the recovery of desired products, but results in the conversion of reactants to commercially uninteresting by-products.
More recent research has centered upon limiting the range of hydrocarbons produced. In most cases, research has centered on improving the selectivity to C.sub.2 to C.sub.4 range .alpha.-olefins. One approach has been to use a zeolite support to limit the hydrocarbon fraction obtained. U.S. Pat. Nos. 4,298,695 and 4,340,503 are representative of such processes. A second approach has been to employ promoters or selective poisons to limit the product distribution. See, e.g., U.S. Pat. Nos. 4,256,654, 4,242,234, and 4,172,842 wherein potassium compounds, such as potassium carbonate, are used in conjunction with an ironcontaining catalyst.
Other additives which have been used include manganese, magnesium, and alkali. Catalyst systems employing nickel, cobalt, and copper have also been reported.
The effect of zinc on iron catalysts has also been investigated, but with contradictory results. Yang and Oblad (preprints, Div. Pet. Chem., ACS meeting, Anaheim, Mar. 12-17, 1978) report that ZnO had no effect when added to an iron catalyst. Papadopoulios et al. (Bull. Soc. Chim. France, Pt. 1, 1982) report that when zinc and titanium were added to an iron catalyst, selectivity to .alpha.-olefins was improved. However, the relative contributions of zinc and titanium were not investigated.
It was further known in the art that the reaction of synthesis gas in the presence of a catalyst system comprising palladium and zinc oxide produced methanol to the virtual exclusion of hydrocarbons. Therefore, this combination of catalyst components would have been deemed to be a poor choice for the production of .alpha.-olefins.
Thus, while several varying approaches to the problem of selective production of .alpha.-olefins from synthesis gas have been reported, none has proven to be entirely satisfactory. It has now been found that .alpha.-olefins can be selectively produced from synthesis gas at advantageous rates by the use of a catalyst system which comprises palladium, iron, and zinc.